Justice Department and law enforcement officials announce indictments to stop fentanyl and other opiate substances from entering the United States on Oct. 17. (Saul Loeb/AFP/Getty Images)

The package Trevor Harden aimed to pick up when he walked into the small brick post office in Chamberlain, S.D., contained tens of thousands of fentanyl pills, authorities said.

Harden had, prosecutors said, ordered 40,000 pills off the dark Web and had them shipped to Chamberlain, a town of 2,300 on the Missouri River. The pills were intercepted in June by postal inspectors, who tipped off authorities.

Postal inspectors like the ones in Rapid City, S.D., who found the package, have become unlikely front-line responders to the opioid crisis. People increasingly are ordering drugs — including fentanyl, which is driving a surge in overdose deaths — via the Internet. Most of the drugs are synthetic, made in China and are shipped in small envelopes or packages.

“Via the Internet you order fentanyl, then it’s mailed over,” said Melvin Patterson, a Drug Enforcement Administration spokesman.

Harden was arrested and pleaded guilty to charges of attempting to possess and intending to distribute fentanyl; his defense attorney declined to comment. His case is part of a widespread crackdown on the use of the mail to traffic in illicit narcotic sales.

It has become such an issue that President Trump’s commission on the opioid crisis has recommended enhanced screening of the mail with drug-sniffing dogs and scrutiny of electronic data, part of a broad set of recommendations it said are needed to curb the opioid epidemic.

“The detection of fentanyl and its analogues shipped directly into the United States via international mail and express consignment presents a unique challenge,” the commission report said.

Part of the difficulty, authorities said, is that small quantities are being sent through the mail because users don’t need much of it to get the euphoric high they are seeking. Fentanyl and its analogues, which are different formulas of the drugs concocted to skirt U.S. drug laws, are incredibly potent — just a few granules can be deadly. Many of the shipments coming through the mail are destined for small-scale dealers and are in small quantities that arrive in small packages that can be difficult to screen.

“What’s coming through the mail is for smaller traffickers and users on the dark Web,” said James Hunt, head of the New York Division of the DEA. “The cartels don’t have to send fentanyl through the mail.”

The sale of drugs on the dark Web was thought to have been slowed in 2013, when Silk Road, a drug marketplace, was shut down. But other sites have proliferated, and profited, in the past five years.

This is an example of the amount of fentanyl that can be deadly, officials say. (Jacquelyn Martin/AP)

The Justice Department is now moving against online dealers. In July, it announced that it shuttered AlphaBay, an illicit Internet market for drugs, firearms and fake documents. It contained more than 250,000 listings for illegal drugs, authorities said. Last month, prosecutors charged two Chinese men who sold fentanyl online to Americans. In May, the U.S. attorney in Utah charged six people with buying fentanyl and other drugs via the Internet from China.

Attorney General Jeff Sessions visited John F. Kennedy Airport in New York last month, where drug dogs and law enforcement officials regularly try to interdict fentanyl.

“Fentanyl is the number one killer drug in America,” Sessions said. “More than 20,000 Americans died of overdoses involving fentanyl last year. And as deadly as it is, you can go online and order it through the mail.”

Sessions said at least 80 packages of fentanyl were intercepted at JFK in the past fiscal year. According to the White House commission, the U.S. Postal Service processed more than 275 million packages from foreign countries destined for the United States in fiscal 2016. There were 2,439 cases of drug trafficking that year, including 39 cases of fentanyl or fentanyl-related drugs, along with 89 pounds of heroin and 13,968 OxyContin pills.

The commission has called for expanding the use of advanced electronic data (AED) — including the names and addresses of senders and recipients — on shipments from overseas, as well as employing more drug-sniffing dogs at postal facilities and points of entry for mail.

“The increase in the percentage of inbound items with AED is expected to continue to grow, especially as we increase our partnership with commercial providers and more countries develop their capability to provide this data,” David Partenheimer, a Postal Service spokesman, said in a statement. “The Postal Service is fully engaged in the concerted efforts of the federal government to confront the opioid epidemic, and we will provide whatever resources are available to us to assist in those efforts.”

The commission also called for passage of a bill sponsored by Sen. Rob Portman (R-Ohio) that would require shippers from foreign countries to submit electronic data before a package is mailed.

“We have a heroin and prescription drug epidemic in our country, and this crisis is being made worse by an influx of deadly synthetic drugs coming into our states from places like China and India,” Portman said in a statement.

In the South Dakota case, the pills were ordered on the Internet, most likely from China, and were routed through California before coming to South Dakota, authorities said. According to court documents, police in Chamberlain received information that Harden was being mailed fentanyl; the postal inspector later intercepted packages mailed to him. Officials picked up a second package they say was bound for Harden; it also contained 20,000 fentanyl pills.

Nick Miroff and Alice Crites contributed to this report.